Power factor (PF) is a parameter used for measuring power consumption efficiency of electrical equipment. To improve the power consumption efficiency, PFC is usually performed to an Alternating Current (AC) input signal before the AC input signal is supplied to the electrical equipment. A PFC device generally increases PF by reducing a phase difference between a voltage and a current.
In existing techniques, a PFC device employs a conventional bridge boost circuit. However, there is a fixed voltage drop in a diode in a rectifier bridge in the circuit, which results in a large loss. As a result, efficiency of the circuit cannot meet requirements.
To overcome the above problems, another existing PFC device employs a bridgeless double boost circuit. However, the circuit requires two sets of boost circuits, which may cause high cost and a relatively large space. In addition, a switching transistor in the circuit is still hard switching, thus, improvement in efficiency is limited.
At present, there is another PFC device where a totem pole circuit is employed. The totem pole circuit uses fewer components to provide increased power density. In addition, a switching transistor in the totem pole circuit is soft switching, so that efficiency is further improved, which is helpful to improve efficiency of the overall machine. Previous totem pole circuits use conventional MOS transistors. Due to reverse recovery effect of a body diode in the conventional MOS transistor, the previous totem pole circuit usually only work in a discontinuous current mode, so that a control strategy is relatively complex. A GaN-based MOS transistor has a higher opening rate and less reverse recovery effect, therefore, a totem pole circuit using the GaN-based MOS transistor can work in the continuous current mode, and thus is prone to be realized.
FIG. 1 schematically illustrates a totem pole circuit in the exiting techniques. Referring to FIG. 1, the circuit includes an inductor L, a first bridge arm 11, a second bridge arm 12 and an output capacitor C. The first bridge arm 11 includes switching transistors Q1 and Q2 which are connected at a first connection point A. The second bridge arm 12 includes switching transistors Q3 and Q4 which are connected at a second connection point B. Further, the switching transistors Q1 and Q2 are primary transistors for controlling charging and discharging of the totem pole circuit, and generally are GaN-based MOS transistors. The switching transistor Q3 is turned on during a positive half-cycle of an AC input signal AC, and the switching transistor Q4 is turned on during a negative half-cycle of the AC input signal AC cycle, so as to perform synchronous rectification.
Referring to FIGS. 2A to 2F, an operation process of the totem pole circuit as shown in FIG. 2 is described below. Referring to 2A, during the positive half-cycle of the AC input signal AC, the switching transistors Q2 and Q4 are turned on, and the switching transistors Q1 and Q3 are turned off. The AC input signal AC is applied to the inductor L, to make an inductor current flowing through the inductor L increased. Referring to 2B, afterwards, the switching transistor Q2 is turned off. Due to a dead time, the switching transistor Q1 is not turned on yet, and the inductor current cannot change suddenly, thus, the current flows though a body diode of the switching transistor Q1 to the output capacitor C. Referring to FIG. 2C, after the dead time, the switching transistor A1 is softly turned on, the inductor L discharges, and the inductor current decreases. During the negative half-cycle of the AC input signal AC, equivalent circuits of the totem pole circuit are shown as FIGS. 2D and 2F. The operation states in the negative half-cycle are similar with the positive half-cycle, only the transistors used for charging and the transistors used for discharging are interchanged, and the synchronous rectification transistor is changed from the switching transistor Q4 to the switching transistor Q3.
To control switching transistors in a totem pole circuit, a control solution is disclosed in a patent with a publication number No. CN101707441B. Two current sampling units are added in a first bridge arm connected with an inductor, where current sampled by the two current sampling units are used for controlling the turn-on and turn-off of the two transistors in the first bridge arm respectively. However, in the patent, one current sampling unit is floating ground connected, that is, the current sampling unit is connected with the ground without through a conductor. Therefore, the circuit may be prone to be affected by parasitic parameters, and inductive interference may be brought to an analog circuit. In addition, the current sampling unit always employs components with great inductance and capacitance to ensure relatively high precision, which causes the current sampling unit to generate parasitic parameters that impact operation effect of the circuit.
Another solution for controlling switching transistors in a totem pole circuit is disclosed in a patent application with publication No. US2012/0293141A1. Specifically, a current transformer is used to detect a current so as to control the switching transistors. However, the current transformer may still bring parasitic parameters that impact the circuit, for example, parasitic inductance. At the moments that the switching transistors are turned on and turned off, the parasitic inductance may generate a relatively large voltage spike, which may cause the circuit to not operate normally.